Not surprisingly, it looks like WordPress.com in their attempt to be as spam free as possible has reminded everyone that PayPerPost advertisements are not allowed on WordPress.com blogs.
WordPress.com’s Terms of Service are very clear about this, and even highlight the important text in blue.
the Content is not spam, and does not contain unethical or unwanted commercial content designed to drive traffic to third party sites or boost the search engine rankings of third party sites, or to further unlawful acts (such as phishing) or mislead recipients as to the source of the material (such as spoofing);
WordPress is trying to create a quality service and quality brand, and they believe that PayPerPost is only a way to try to game search engines. So, if you want to use such a service, head somewhere else because at this point, WordPress.com doesn’t want you to post it there.